qLTG-9, a stable quantitative trait locus for low-temperature germination in rice (Oryza sativa L.).

Low-temperature germination (LTG) is an important agronomic trait for direct seeding of rice in temperate regions of East Asia. To dissect the genetic control of LTG, we constructed a recombinant inbred line (RIL) population derived from a cross of japonica variety USSR5 and indica variety N22. Three putative QTL involved in LTG were detected and named qLTG-7, qLTG-9 and qLTG-12. They explained 9.5, 12.12 and 7.08 % of the phenotypic variation, respectively, and the alleles from USSR5 enhanced LTG. A set of advanced backcross lines selected for the presence of qLTG-9 (with the biggest contribution of the three QTL), by both linked markers and phenotype, was used to validate qLTG-9 in different generations, years and locations. A near-isogenic line in USSR5 background with a qLTG-9 insertion from N22 had retarded germination under low-temperature conditions. Finally, qLTG-9 was fine mapped between markers L9-25D and ID-1, to a 72.3-kb region in chromosome 9, which in the Nipponbare genome contains five predicted genes. This result provides a springboard for map-based cloning of qLTG-9 and is helpful in understanding the mechanism of seed germination under low-temperature conditions.